FIX: Set default window size to 20

[pjyi2147]

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ref: #144

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[pjyi2147]

Another suggestion:
Once a paremeter defined in **kwargs in any way (even None), it is no use of setting default values in kwargs.get().
It would be more clear to remove default values in kwargs.get() and let it fail if similar error occurs.

Example code for top_k_retrieve and window_size.

They are defined in run_rank_llm.py, therefore, no default values are required.

# rank_listwise_os_llm.py:116-117
top_k_retrieve: int = kwargs.get("top_k_retrieve", 50)
window_size: int = kwargs.get("window_size", 20)